Tribological Characterisation of Lubricant Additives – Friction and Wear Behaviour
Background
The basic function of a lubricant is to reduce friction and wear between two surfaces in relative motion. This is usually achieved by forming a load-bearing fluid film at the contact interface. However, in many applications, the role played by lubricants is far more complex. For instance, in addition to the basic tasks mentioned above, lubricant is used in automotive engines to dissipate heat, reduce wear, carry foreign particles away from the mating interface, prevent oxidation and corrosion of the metal parts, maintain stability at high temperature, provide effective sealing, to name a few. All these requirements, however, cannot be fulfilled by the base oil alone. Therefore, specific additives are used, which address these explicit requirements. A typical composition of engine oil used in diesel engines is presented below in Fig. 1. While each of the additives is expected to fulfil its designated function, it is important that any interactions between the additives themselves do not have a negative impact on the overall performance of the lubricant.
The demand for increased efficiency and the need to conform to the environmental norms and regulations are two factors driving the lubricant industry to improve their products. This however is easier said than done as lubricants are generally a finely balanced composition of numerous chemical components. With almost every tweak in the formulation of a lubricant, there is a fair chance of casting a significant influence on its friction and wear behaviour. This influence can either be positive or negative and therefore, oil formulations are thoroughly investigated before they find their way onto the market.
Tests on lubricants are generally carried out at different scales, such as field tests, engine bench tests, component tests, model tests, etc. While a field test provides the best transferability of results, it is quite cost and time intensive. Furthermore, owing to the complexity of the system, evaluation and interpretation of the results is also quite cumbersome. On the other hand, model scale tests are reasonably cost effective and less time consuming. They can also slow down processes or events leading to failure of the system, thus enabling the user to observe and study individual failure mechanisms. Tests at model scale can be used to screen materials for particular applications and also characterize the effect of individual components of the system on its overall performance.
Experimental setup The tribological tests were carried out on an MCR Tribometer, MCR 502, from Anton Paar. The test configuration used here was ball-on-three-pins, see Fig. 2, employing a Peltier heated tribology cell T PTD 200 in combination with a Peltier hood H-PTD 200 for precise temperature control. The wear-scar measurements on the tested specimen were carried out on an Olympus BX51M light microscope.
Figure 1. Typical composition of lubricant used in automotive engines.
A set of specimens for each of the tests consists of one ball and three pins. 100Cr6 steel was used for manufacturing the ball and the pins. While the ball measured 12.7 mm in diameter, the
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LUBE MAGAZINE NO.130 DECEMBER 2015
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